The long range goal of this proposal is to examine the interaction between regular exercise and the course and outcome of the pregnancy. Its specific aims are: to determine if regular exercise induces changes in maternal physiological function and placental growth and functional capacity and thereby improve fetal growth and pregnancy outcome in a variety of populaces, to assess the modulatory role of diet on these exercise effects, and to determine if the beneficial effects of exercise are regulated in part by decreasing the need for flow redistribution during cardiovascular stress. It will test 3 hypotheses. First, that beginning a program of regular exercise early in the second trimester will reduce the incidence of preterm birth and low birth weight in demographically at risk populaces. Second, that dietary carbohydrate mix modifies the impact of exercise on maternal weight gain, fat deposition, feto-placental growth and size at birth. Third, different threshold volumes of regular exercise are required to produce each specific beneficial effect. It is based on earlier studies which indicate that regular exercise during pregnancy has positive effects for both mother and fetus in two specific populaces of women. It will complete prospective, randomized training studies in two groups of women who are at low demographic risk and in two groups of women who are at high demographic risk for poor outcome. The techniques employed include: respiratory calorimetry and doubly labeled water for energy expenditure, ultrasound for measuring feto-placental growth, total body electrical conductivity and 18/O for body composition, along with other gross and histomorphometric techniques. The proposal should determine the preventive value of regular exercise in both low and high risk pregnancy and provide insight into several mechanisms which regulate its beneficial effects.